Solubility Equilibria Of Sparingly Soluble Salts
The maximum amount of the solute that dissolves in 100 ml of the solvent at a given temperature and pressure is called solubility. The ions in an ionic solid are held together by strong electrostatic forces of attraction. For its dissolution in the solvent , energy is required to overcome the attractive forces between the ions. This energy is called the lattice enthalpy. The Lattice enthalpy of an ionic solid is defined as the energy required to completely separate one mole of a solid ionic compound into its constituent ions. When an ionic solid is dissolved in a solvent, certain amount of energy is released . The amount of energy released when one mole of a solid salt is dissolved in a solvent is called the solvation enthalpy. A salt is soluble in a solvent only when the solvation enthalpy is higher than the lattice enthalpy of the salt. Ex: Sodium chloride is highly soluble in polar solvent water because the solvation enthalpy of water is sufficiently high enough to overcome the lattice enthalpy of the sodium chloride salt. Kerosene and benzene are non-polar solvents. Their solvation enthalpy is small and not sufficient enough to overcome the lattice enthalpy of the salt. Hence, salts do not dissolve in non-polar solvents. Depending on the extent of solubility, salts are classified into three categories. Salts with solubility greater than 0.1 moles/litre are considered to be soluble. Salts with solubility between 0.01 moles and 0.1 moles/ litre are considered to be slightly soluble. And, salts with solubility less than 0.01 moles/ litre are considered to be sparingly soluble. The solubility product of a sparingly soluble salt at a given temperature may be defined as the product of the concentrations of its ions in the saturated solution with the concentration terms given as Ksp = [A+]1[B-]1 AB(s) = 1A+(aq) + 1B-(aq) The expression for solubility product (Ksp) can be written as Ex: For a salt of MX2 type  AX2 A+2 + 2X- [A+2] = S [X-] = 2S  Ksp = [A+2]  Ksp = S x (2S)2  Ksp = 4S3 A salt may give two or more than two cations and anions when dissolved in water. Ex: Solubility product of Calcium phosphate When calcium phosphate is dissolved in water, it dissociates into three calcium ions and two phosphate ions.           Ca3(PO4)2 $⇌$ 3Ca+2 + 2PO4-3                      Ksp =                      [Ca+2] = 3S                      [PO-34] = 2S                         Ksp = (3S)3(2S)2                                = 27S3 x 4S2     The Solubility of the Product of Calcium Phosphate = 108 x S5

#### Summary

The maximum amount of the solute that dissolves in 100 ml of the solvent at a given temperature and pressure is called solubility. The ions in an ionic solid are held together by strong electrostatic forces of attraction. For its dissolution in the solvent , energy is required to overcome the attractive forces between the ions. This energy is called the lattice enthalpy. The Lattice enthalpy of an ionic solid is defined as the energy required to completely separate one mole of a solid ionic compound into its constituent ions. When an ionic solid is dissolved in a solvent, certain amount of energy is released . The amount of energy released when one mole of a solid salt is dissolved in a solvent is called the solvation enthalpy. A salt is soluble in a solvent only when the solvation enthalpy is higher than the lattice enthalpy of the salt. Ex: Sodium chloride is highly soluble in polar solvent water because the solvation enthalpy of water is sufficiently high enough to overcome the lattice enthalpy of the sodium chloride salt. Kerosene and benzene are non-polar solvents. Their solvation enthalpy is small and not sufficient enough to overcome the lattice enthalpy of the salt. Hence, salts do not dissolve in non-polar solvents. Depending on the extent of solubility, salts are classified into three categories. Salts with solubility greater than 0.1 moles/litre are considered to be soluble. Salts with solubility between 0.01 moles and 0.1 moles/ litre are considered to be slightly soluble. And, salts with solubility less than 0.01 moles/ litre are considered to be sparingly soluble. The solubility product of a sparingly soluble salt at a given temperature may be defined as the product of the concentrations of its ions in the saturated solution with the concentration terms given as Ksp = [A+]1[B-]1 AB(s) = 1A+(aq) + 1B-(aq) The expression for solubility product (Ksp) can be written as Ex: For a salt of MX2 type  AX2 A+2 + 2X- [A+2] = S [X-] = 2S  Ksp = [A+2]  Ksp = S x (2S)2  Ksp = 4S3 A salt may give two or more than two cations and anions when dissolved in water. Ex: Solubility product of Calcium phosphate When calcium phosphate is dissolved in water, it dissociates into three calcium ions and two phosphate ions.           Ca3(PO4)2 $⇌$ 3Ca+2 + 2PO4-3                      Ksp =                      [Ca+2] = 3S                      [PO-34] = 2S                         Ksp = (3S)3(2S)2                                = 27S3 x 4S2     The Solubility of the Product of Calcium Phosphate = 108 x S5

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